Composite Chalcogenide Materials and Devices

ABSTRACT

An electrical device includes a composite switching material. The composite switching material includes an electrically switchable component and a non-switchable component. In one embodiment, the composite switching material includes a heterogeneous mixture of at least one chalcogenide material and at least one dielectric material. The composite switching material is disposed between two electrodes and the switchable component is transformable from a resistive state to a conductive state upon application of a voltage between the two electrodes, without changing phase.

RELATED APPLICATIONS

The present application is a continuation in part of U.S. Reissueapplication Ser. No. 10/190,858, titled “MEMORY ELEMENT WITH MEMORYMATERIAL COMPRISING PHASE-CHANGE MATERIAL AND DIELECTRIC MATERIAL,”filed on Jul. 8, 2002, being a Reissue of application Ser. No.09/063,174, titled “MEMORY ELEMENT WITH MEMORY MATERIAL COMPRISINGPHASE-CHANGE MATERIAL AND DIELECTRIC MATERIAL,” filed on Apr. 20, 1998,which is now issued as U.S. Pat. No. 6,087,674, which is also acontinuation in part of U.S. application Ser. No. 08/739,080, titled“MEMORY ELEMENT WITH MEMORY MATERIAL COMPRISING PHASE-CHANGE MATERIALAND DIELECTRIC MATERIAL,” filed on Oct. 28, 1996, which is now issued asU.S. Pat. No. 5,825,046, wherein the contents of the above mentionedapplications are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The embodiments described herein are generally directed to electricaldevices including a composite switching material. More particularly, theinvention is related to electrical devices including a compositechalcogenide switching material.

BACKGROUND

Chalcogenide switching devices are used, for example, with phase-changememory devices as an access element. The phase-change non-volatilememory devices are beneficial in applications where data must beretained when power is disconnected. Applications include general memorycards, consumer electronics (e.g., digital camera memory), automotive(e.g., electronic odometers), and industrial applications (e.g.,electronic valve parameter storage). The non-volatile memories may usephase-change memory materials, i.e., materials that can be switchedbetween a generally amorphous and a generally crystalline state, forelectronic memory applications. The memory of such devices typicallycomprises an array of memory elements, each element defining a discretememory location and having a volume of phase-change memory materialassociated with it. The structure of each memory element typicallycomprises a phase-change material, one or more electrodes, and one ormore dielectrics.

One type of memory element originally developed by Energy ConversionDevices, Inc. utilizes a phase-change material that can be, in oneapplication, switched between a structural state of generally amorphousand generally crystalline local order or between different detectablestates of local order across the entire spectrum between completelyamorphous and completely crystalline states. These different structuralstates have different values of resistivity, and therefore each statecan be determined by electrical sensing. Typical phase-change materialssuitable for memory application include those incorporating one or morechalcogen or pnictogen elements. Unlike certain known devices, theseelectrical memory devices typically do not use field-effect transistordevices as the memory storage element. Rather, they comprise, in theelectrical context, a monolithic body of thin film chalcogenidematerial. As a result, very little area is required to store a bit ofinformation, thereby providing for inherently high-density memory chips.

Ovonic unified or phase-change memories are an emerging type ofelectrically-alterable non-volatile semiconductor memories. Thesememories exploit the properties of materials (phase-change materials)that can be reversibly switched between two or more structural statesthat vary in the relative proportions of amorphous and crystalline phaseregions when subjected to heat or other forms of energy. The term“amorphous” refers to a condition which is relatively structurally lessordered or more disordered than a single crystal and has a detectablecharacteristic, such as high electrical resistivity. The term“crystalline” as used herein refers to a condition which is relativelystructurally more ordered than amorphous and has at least one detectablydifferent characteristic, such as a lower electrical resistivity.

The distinct structural states of a phase-change material exhibitdifferent electrical characteristics, such as resistivity, that can beused to distinguish the different states. Memory or logic functionalityis achieved by associating a different memory or logic value with eachstructural state. Programming occurs by providing the energy needed tostabilize the structural state of the phase-change material associatedwith the input memory or logic data.

Typically, a memory array includes a matrix of phase-change memorycells, arranged in rows and columns with associated word lines and bitlines, respectively. Each memory cell typically consists of aphase-change storage element connected in series to an access element,where each memory cell is connected between a particular word line and aparticular bit line of the array. Each memory cell can be programmed toa particular memory state by selecting the word line and bit lineassociated with the memory cell and providing a suitable energy pulseacross the memory cell. The energy pulse is typically a current pulseapplied to the memory cell by supplying a voltage potential between theword line and bit line of the cell. The voltage potential activates theaccess element connected to the memory element, thereby enabling theflow of current through the memory element. Typical access elementsinclude diodes and transistors. Reading of the memory state isaccomplished by similarly selecting the word line and bit line of thememory cell and measuring the resistance (or a proxy therefor (such asthe voltage drop across the cell). In order to maintain the state of thememory cell during read, it is necessary to maintain the energy of theread signal at a level below that needed to transform the memory cellfrom its existing state to a different state.

In addition to memory elements, switching elements, particularly fastswitching devices, are desirable for a number of applications. Fastswitching elements are capable of being switched between a relativelyresistive state and a relatively conductive state and are useful asvoltage clamping devices, surge suppression devices, and signal routingdevices. Fast switching elements can also be used as access devices inmemory arrays.

An important class of fast switching materials are the Ovonic ThresholdSwitch (“OTS”) materials. OTS materials, like many phase-change memorymaterials, typically include one or more chalcogen elements. Unlikephase-change memory materials, however, the compositions of OTSmaterials are such that no change in structural state occurs within therange of normal operation of the material. Instead, the OTS materialretains an overall predominantly amorphous structure during operation.Application of a suitable energy signal, typically an electrical energysignal having a voltage above a critical threshold level, induces achange of the electrical characteristics of the OTS device from arelatively resistive quiescent off state to a relatively conductive onstate. The relatively conductive state persists for so long as thecurrent passing through the OTS material remains above a criticalholding level. Once the energy signal is removed or the currentotherwise decreases below the level needed to sustain the relativelyconductive on state, the OTS material relaxes back to its relativelyresistive quiescent off state.

Under one theory of operation, an OTS material achieves its conductiveon state through the formation of a localized, conductive filamentaryregion that extends across the material between opposing electricalcontacts when the voltage applied between the contacts is at or above athreshold voltage. When the current across the material is decreased tobelow the holding level needed to sustain the conductive state, thefilamentary region collapses and the material switches back to itsresistive quiescent state. As the material is switched between itsresistive and conductive states over multiple cycles of operation, thefilamentary region is repeatedly formed and extinguished.

A consequence of this mechanism of operation is that the reproducibilityand stability of the switching event over multiple cycles of operationdepends on the consistency of the characteristics of the localizedfilament. Optimal performance requires consistent physical placement ofthe filament within the OTS material and a reproducible thresholdvoltage to insure control over initiation of the switching event. It isalso necessary for the holding current to remain stable over multiplecycles of operation. In practice, it has been observed that thethreshold voltage, holding current, and/or physical placement of thefilamentary region of OTS materials may vary upon cycling and theswitching characteristics of OTS materials is accordingly compromised.

One strategy for stabilizing the operation of OTS materials is toenlarge the lateral dimensions of the material in the switching deviceand use large area contacts. This approach tolerates variations in thephysical location of the filament upon cycling by insuring the existenceof an adequate voltage for switching over a range of positions withinthe material. As a result, any variability in the stabilized position ofthe filament that may occur upon repeated cycling is accommodated andthe possibility of a failure to switch is minimized. The drawback tothis approach, however, is that large area contacts increase the energyrequired to sustain the off state by facilitating leakage of currentthrough non-switched OTS devices leading to high standby current inproduct applications. Large area devices are further contrary to thegeneral goal of increasing device density in order to reduce cost.

There is accordingly a need for switching devices that utilize smallarea contacts (to reduce the energy required to sustain the off state),while providing for consistent switching performance over multiplecycles of operation.

SUMMARY

An electrical switching device includes a composite material. Thecomposite material includes a heterogeneous mixture of at least onechalcogenide material and at least one dielectric material. Theelectrical device may further include a first electrode in electricalcommunication with the composite switching material and a secondelectrode in electrical communication with the composite switchingmaterial.

In an alternative embodiment, an electrical device includes a lowerelectrode, a lower insulator above the first electrode and a 2^(nd)lower electrode through said lower insulator and in electricalcommunication with the lower electrode. The electrical device furtherincludes a composite switching material in electrical communication withthe 2^(nd) lower electrode and an upper electrode in electricalcommunication with the composite switching material.

In addition to use as a switching device, applications of the compositeswitching device include use as an access element for memory cells in aphase-change electrical memory array.

Further, a method of making an electrical device is disclosed. The stepsinclude depositing a lower electrode and co-depositing at least onechalcogen element and at least one dielectric material. An upperelectrode is also deposited.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and inventive aspects will become more apparent uponreading the following detailed description, claims, and drawings, ofwhich the following is a brief description:

FIG. 1 is a schematic I-V characteristic of a chalcogenide materialexhibiting an electrical switching transformation.

FIG. 2 is a cross-sectional view of a composite chalcogenide device.

FIG. 3 is a plan-view of a 2^(nd) lower electrode of FIG. 2.

FIG. 4 is a cross-sectional view of a composite chalcogenide layer foruse with the composite chalcogenide device of FIG. 2.

FIG. 5A is a cross-sectional view of the composite chalcogenide deviceof FIG. 2 having a first filament formed in the composite chalcogenidematerial having relatively large insulative particles.

FIG. 5B is a cross-sectional view of the composite chalcogenide deviceof FIG. 2 having a second filament formed in the composite chalcogenidematerial having relatively large insulative particles.

FIG. 6A is a cross-sectional view of the composite chalcogenide deviceof FIG. 2 having a first filament formed in the composite chalcogenidematerial having relatively small insulative particles.

FIG. 6B is a cross-sectional view of the composite chalcogenide deviceof FIG. 2 having a second filament formed in the composite chalcogenidematerial having relatively small insulative particles.

FIG. 7A is a cross-sectional view of the composite chalcogenide deviceof FIG. 2 having a first filament formed in the composite chalcogenidematerial having very small insulative particles

FIG. 7B is a cross-sectional view of the composite chalcogenide deviceof FIG. 2 having a second filament formed in the composite chalcogenidematerial having very small insulative particles.

FIG. 8 is a graph describing leakage current for the compositechalcogenide device of FIG. 2.

FIG. 9 is a graph describing an example of life cycle for the compositechalcogenide device of FIG. 2.

FIG. 10 is a flow diagram of the construction of the compositechalcogenide device of FIG. 2.

DETAILED DESCRIPTION

Referring now to the drawings, illustrative embodiments are shown indetail. Although the drawings represent the embodiments, the drawingsare not necessarily to scale and certain features may be exaggerated tobetter illustrate and explain novel aspects of an embodiment. Further,the embodiments described herein are not intended to be exhaustive orotherwise limit or restrict the claims to the precise form andconfiguration shown in the drawings and disclosed in the followingdetailed description.

A composite switching device is described in detail herein. Thecomposite switching device includes a composite switching materialdisposed between two or more electrodes and otherwise insulated fromsurrounding structures. The composite switching material includes anactive switching material interspersed with an electrically inert ornon-switchable material in the operable region between the electrodes ofthe device. The active switching material can be switched between two ormore states that differ in resistivity. In one embodiment, the activeswitching material is switchable from a relatively resistive state to arelatively conductive state without undergoing a change in phase.Representative active switching materials include chalcogenide materialsand OTS materials. The non-switchable component of the compositeswitching material is typically a dielectric material. The scope of theinstant invention extends to electrical switching devices having thefull range of relative proportions of the active switching material andnon-switchable material within the volume of the operable portion of thedevice between the electrodes.

When a dielectric material is introduced with a chalcogenide switchingmaterial to form a composite switching material, the operationalparameters of the switching event are stabilized relative tocorresponding devices that include a homogenous chalcogenide switchingmaterial without a non-switchable component in the region between theelectrodes of the device. Stabilization of the switching parameters isaccomplished at least in part because the dielectric material in thecomposite chalcogenide material is mechanically harder than thechalcogenide switching material. The high mechanical strength of thedielectric renders it less susceptible to mechanical deformation duringoperation and cycling of the device. The dielectric material thusrigidly confines the physical location of the chalcogenide switchingmaterial within the device and minimizes the tendency for physicalmotion, rearrangement, or positional variation of the chalcogenideswitching material during operation and cycling of the device. Theresulting stabilization of the chalcogenide switching material allowsfor improved repeatability and stability of the threshold voltage(V_(th)), threshold current (I_(th)), holding voltage (V_(h)), and/orholding current (I_(h)) of the device.

The composite chalcogenide switching material provides reduction ofleakage current because the presence of the dielectric reduces theavailability of alternative pathways to current flow due to its highresistivity. When a voltage is applied across the electrodes inelectrical communication with a composite chalcogenide switchingmaterial, the low resistivity of the chalcogenide material when in theconductive state relative to the dielectric material insures thatcurrent flow will occur primarily through the chalcogenide. The highresistivity of the dielectric component of the composite switchingmaterial inhibits conduction and current flows in pathways around thedielectric component. The available current pathways in the operableregion between opposing electrodes are thus largely defined by thephysical location and distribution of the chalcogenide component of thecomposite switching material. The net result is a localized andreproducible channeling of current through the chalcogenide component.For the case of the OTS device in the resistive state, leakage currentis minimized and the required energy of operation during standby oractive operation is reduced and greater efficiency is achieved. Becauseof the more rigid confinement of the switchable component by thedielectric, the filamentary region formed in embodiments of switchablematerials that progress from a resistive state to a conductive statethrough the formation of a filament is better localized and moreconsistently formed in the same physical location over multiple cyclesof operation. This leads to greater stabilization and reproducibility ofoperating parameters such as threshold voltage and holding current.

Inclusion of the dielectric further facilitates the goal of deviceminiaturization. The presence of the dielectric reduces the overallvolume fraction of the switchable chalcogenide material in the operableregion between the electrodes as well as the area of contact of theswitchable chalcogenide material with the electrodes at the interfacebetween the electrodes and operable region of the device. Since it isunnecessary to supply current to the dielectric component, it becomespossible to reduce the effective dimensions of the device electrodes. Inprinciple, the dimensions of the device electrode need only coincidewith the area of contact of the switchable chalcogenide material at theinterface. Because of the high stability and resistivity of thedielectric surrounding the switchable chalcogenide material, the lateraldimensions of the dielectric component can also be reduced withoutcompromising performance and an overall reduction in the size of thedevice is achieved. Further advantages are described in connection withthe examples presented hereinbelow.

The composite switching material typically consists of a switchablechalcogenide component and a non-switchable dielectric component mixedtogether. The chalcogenide component may be in one single phase,generally amorphous. Particularly, the composite switching device may bea two-terminal device which switches from a high-resistance condition toa low-resistance condition when a voltage applied thereacross or thecurrent therethrough exceeds a threshold value V_(th) at I_(th) (forexample, about 3 V and about 1 μa), and reverts to the high-resistancecondition when a current flowing therethrough falls below a minimumholding value I_(h) (for example, a few microamperes or less).

The electrical switching and current characteristics of a chalcogenideswitching material according to one embodiment of the instant devicesare schematically illustrated in FIG. 1, which shows the I-V(current-voltage) characteristics of a chalcogenide switching material.The I-V characteristics depicted in FIG. 1 may be conveniently describedby considering a simple two-terminal device configuration in which twospacedly disposed electrodes are in contact with a chalcogenide materialand the current I corresponds to the current passing between the twoelectrodes. The I-V curve of FIG. 1 shows the current passing through achalcogenide switching material as a function of the voltage appliedacross the material by the electrodes. The I-V characteristics of thematerial are generally or approximately symmetric with respect to thepolarity of the applied voltage. For convenience, we consider the firstquadrant of the I-V plot of FIG. 1 (the portion in which current andvoltage are both positive) in the brief discussion of chalcogenideswitching behavior that follows. An analogous description that accountsfor polarity applies to the third quadrant of the I-V plot.

The I-V curve of a chalcogenide switching material includes a resistivebranch 10 and a conductive branch 20 as shown in FIG. 1. Resistivebranch 10 corresponds to a branch in which the current passing throughthe material increases only slightly upon increasing the voltage appliedacross the material. Resistive branch 10 exhibits a small slope in theI-V plot and appears as a more nearly horizontal line in the first andthird quadrants of FIG. 1. Conductive branch 20 corresponds to a branchin which the current passing through the material increasessignificantly upon increasing the voltage applied across the material.Conductive branch 20 exhibits a large slope in the I-V plot and appearsas a more nearly vertical line in the first and third quadrants ofFIG. 1. The particular slopes of the resistive branch 10 and conductivebranch 20 shown in FIG. 1 are illustrative and not intended to belimiting, the actual slopes will depend on the chemical composition,thickness etc. of the chalcogenide material as well as on parameterssuch as the resistance, capacitance etc. of surrounding circuitelements. Regardless of the actual slopes, conductive branch 20necessarily exhibits a larger (steeper) slope than resistive branch 10.When device conditions are such that the chalcogenide material isdescribed by a point on resistive branch 10 of the I-V curve, thechalcogenide material or device may be said to be in a resistive state.When device conditions are such that the chalcogenide material isdescribed by a point on conductive branch 20 of the I-V curve, thechalcogenide material or device may be said to be in a conductive state.

The capacity of a chalcogenide material in accordance with the instantinvention to carry a current can be described by reference to FIG. 1. Weinitially consider a two-terminal device configuration in which novoltage difference is present between the terminals. When no voltage isapplied across the chalcogenide material, the material is in a resistivestate and no current flows. This condition corresponds to the origin ofthe I-V plot shown in FIG. 1. The chalcogenide remains in a resistivestate as the applied voltage is increased, up to a threshold voltage(labeled V_(th) in the first quadrant of FIG. 1). The slope of the I-Vcurve for applied voltages between 0 and V_(th) is small in magnitudeand indicates that the chalcogenide material has a high electricalresistance, a circumstance reflected in the terminology “resistivebranch” used to describe this portion of the I-V curve. The highresistance implies low electrical conductivity and as a result, thecurrent flowing through the material increases only weakly as theapplied voltage is increased.

When the applied voltage equals or exceeds the threshold voltage, thechalcogenide material switches from resistive branch 10 to conductivebranch 20 of the I-V curve. The switching event occurs almostinstantaneously and is depicted by the dashed line in FIG. 1. Uponswitching, the device voltage decreases significantly and the devicecurrent becomes much more sensitive to changes in the device voltage.The chalcogenide material remains in conductive branch 20 as long as aminimum current, labeled I_(h) in FIG. 1, is maintained. We refer toI_(h) as the holding current and the associated voltage V_(h) as theholding voltage of the device. If the device conditions are changed sothat the current becomes less than I_(h), the material normally returnsto resistive branch 10 of the I-V plot and requires re-application of athreshold voltage to resume operation on conductive branch 20. If thecurrent is only momentarily (e.g. a time less than the recovery time ofthe chalcogenide material) reduced below I_(h), the conductive state ofthe chalcogenide may be recovered upon restoring the current to or aboveI_(h). Analogous switching behavior occurs in the third quadrant of theI-V plot shown in FIG. 1.

The switching effect of the instant devices originates from atransformation of the chalcogenide material from a resistive state to aconductive state upon application of a threshold voltage, V_(th). Amodel can be used to describe the phenomenon underlying the switchingtransformation. According to the model, application of the thresholdvoltage causes the formation of a conductive channel or filament withinthe chalcogenide material. In a typical device configuration, a voltagedifference is applied across two terminals (electrodes) in electricalcommunication with a chalcogenide material and a conductive filamentforms within the chalcogenide material between the two terminals. At thethreshold voltage, the electric field experienced by portions of thechalcogenide material may be sufficiently high to induce a breakdown oravalanche effect whereby electrons are removed from atoms to form ahighly conductive, plasma-like filament of charge carriers. Rather thanbeing bound to atoms in bonds or lone pair orbitals, some electronsbecome unbound and highly mobile. As a result, a conductive channel orfilament forms. The conductive filament constitutes a conductive portionor volume within the otherwise resistive chalcogenide material. Theconductive filament extends through the chalcogenide material betweenthe device terminals across which the threshold voltage is applied andprovides a low resistance pathway for electrical current between thoseterminals. Portions of the chalcogenide material outside of the filamentremain resistive. Since electric current traverses the path of leastresistance, the presence of a conductive filament renders thechalcogenide material conductive and establishes a conductive state.

The conductive filament is maintained between the device terminals aslong as the device current remains at or above the holding current. Aconductive filament is present for all points along conductive branch20, but the cross sectional area of the filament differs for differentpoints along conductive branch 20. The cross sectional area of thefilament refers to directions lateral to the direction of current flow.Depending on operating conditions within conductive branch 20, thefilament can be narrow or wide. As the applied voltage is increasedalong conductive branch 20, the cross section of the filament isenlarged as the applied current is increased. The enlarged filamentindicates a greater portion or volume of the chalcogenide material is ina conductive state and exhibits high conductivity. As a result, thechalcogenide material can support a greater current, as indicated byconductive branch 20 of the I-V curve, which shows an increase in thedevice current as the voltage difference applied across the terminalsincreases. Variations of the voltage applied to a chalcogenide materialoperating in conductive branch 20 modify the width or thickness of thefilament in directions lateral to the direction of current flow. As aresult, the cross-sectional area of chalcogenide material in aconductive state and current carrying capacity of a chalcogenidematerial increase with increasing voltage along conductive branch 20.

The composite switching device thus has distinct operating regimeshaving high and low resistance demarcated by the threshold voltageV_(th). The composite switching device conducts a low current in theunswitched high resistance regime that occurs before the device issubjected to the threshold voltage V_(th). In the switched, lowresistance state, the device conducts a current at or above the holdingcurrent I_(h) while being maintained at a voltage at or above theholding voltage V_(h). The holding voltage V_(h) is lower than thethreshold voltage V_(th) (for example, about 0.5 V). When the compositedevice switches from the high-resistance condition to the low-resistancecondition, a voltage across the composite device then may snap back fromV_(th) to a lower voltage, V_(h)+I×R(dV/dI), where R(dV/dI) is thedynamic resistance of the composite device after switching (triggeringon).

In general, chalcogen-based devices, such as described herein, exhibitswitching into an ON state (e.g., a low resistance state) when athreshold voltage is exceeded. When the voltage is removed, the devicecurrent falls below the holding current and the device switches to anOFF state (e.g. a higher resistance state than the ON state). Theswitching event relies on formation of a filament through the compositechalcogenide material, in the embodiments described herein as achalcogenide material, to carry current from one electrode to another.However, minor variations in switching voltage and current may beexperienced because a filament formed during a switching operation maytake a different path through the composite chalcogenide material asdescribed hereinabove with each occurrence of a switching event becausethe composite chalcogenide material is not precisely controllable to theatomic level with each ON/OFF switching event.

Examples of chalcogenide materials are found in commonly assigned U.S.Pat. No. 5,166,758, U.S. Pat. No. 5,296,716, U.S. Pat. No. 5,534,711,U.S. Pat. No. 5,536,947, U.S. Pat. No. 5,543,737, U.S. Pat. No.5,596,522, U.S. Pat. No. 5,687,112, U.S. Pat. No. 5,694,146, U.S. Pat.No. 5,757,446, and U.S. Pat. No. 6,967,344. The disclosures of U.S. Pat.Nos. 5,166,758, 5,296,716, 5,534,711, 5,536,947, 5,543,737, 5,596,522,5,687,112, 5,694,146, 5,757,446, and 6,967,344 are incorporated byreference herein.

FIG. 2 is a cross-sectional view of a composite chalcogenide switchingdevice 100. A lower isolation layer 102 carries composite chalcogenideswitching device 100. Lower isolation layer 102 is generally adielectric and may be, for example, a layer of silicon dioxide (SiO₂)(present as a grown oxide on an underlying silicon-based substrate orSiO₂ formed from TEOS (tetraethoxysilane)), other oxides, Si₃N₄, orother nitrides. The lower isolation layer is typically supported by anunderlying substrate or wafer that may include peripheral circuitry suchas transistors, diodes, power supplies etc. A lower electrode 110carries current to a 2^(nd) lower electrode 130. A lower insulator 120surrounds the 2^(nd) lower electrode 130 and provides electrical andthermal insulation to a composite chalcogenide switching layer 150 fromlower electrode 110. A hole 122 through lower insulator 120 holds the2^(nd) lower electrode 130. An upper electrode 170 is in electricalcommunication with composite chalcogenide switching layer 150. A cappinginsulator 190 insulates composite chalcogenide switching device 100 fromother circuitry.

Lower electrode 110 and upper electrode 170 are preferably metal, eachof which may be further connected to external circuits. Becausecomposite chalcogenide device 100 is typically constructed betweenvarious layers of an integrated circuit, insulative structures areprovided for isolation of composite chalcogenide device 100. Electricalisolation is provided for the efficient operation of compositechalcogenide device 100 so that electric current leakage is reduced andundesired interactions with adjacent circuitry or neighboring compositechalcogenide devices are minimized or avoided.

Capping insulator 190 provides thermal and electrical insulation ofupper electrode 170 and composite chalcogenide layer 150 from adjacentcircuits and structures (not shown). Similarly, lower isolation layer102 provides thermal and electrical insulation of lower electrode 110from adjacent structures. Within composite chalcogenide device 100,lower insulator 120 provides thermal and electrical insulation ofcomposite chalcogenide layer 150 from lower electrode 110 except at the2^(nd) lower electrode 130, which delivers current to the chalcogenide.The volume of composite chalcogenide layer 150 generally between the2^(nd) lower electrode 130 and upper electrode 170 defines the activeregion of the device.

Lower isolation layer 102 and capping insulator 190 generally allowcomposite chalcogenide device 100 to be located adjacent tosemiconductor regions or metallization and/or interconnect layers. Suchan arrangement facilitates the placement of composite chalcogenidedevice 100 within the strata of any type of mass-produced layereddevices.

FIG. 3 is a plan-view of a the 2^(nd) lower electrode layer 130 of FIG.2. Hole 122 is formed through lower insulator 120 and the 2^(nd) lowerelectrode 130 is formed by deposition and subsequent CMP therein. The2^(nd) lower electrode layer 130 is a generally a planer layer thatcontacts composite chalcogenide layer 150. Current passes thought the2^(nd) lower electrode to the composite chalcogenide layer.

FIG. 4 is a cross-sectional view of a composite chalcogenide layer 150for use with composite chalcogenide device 100 of FIG. 2. In detail,composite chalcogenide layer 150 has a chalcogenide material 310 and adielectric material 320 interspersed therein. Dielectric material 320 istypically sputtered or co-sputtered onto the 2^(nd) lower electrodelayer 130 (shown in FIG. 3) along with chalcogenide material 310 toprovide a composite layer. For clarity, dielectric material 320 is shownhaving exaggerated particle size (see contrasting particle sizes ofFIGS. 6A-7B). In an embodiment, composite chalcogenide layer 150 is aheterogeneous mixture of a chalcogenide switching material or Ovonicthreshold switching alloy and a dielectric material.

Composite chalcogenide layer 150 is provided as a layer of chalcogenideinterspersed with dielectric material 320 and is in electricalcommunication and thermal communication with the 2^(nd) lower electrode130. Composite chalcogenide layer 150 may be, in an example, aTe₂₈As₄₂Ge₃₀ chalcogenide alloy having silicon dioxide (SiO₂) asdielectric material 320. Composite chalcogenide layer 150, in anexample, includes at least one chalcogenide element selected from Te andSe, and may further include one element selected from the groupconsisting of Ge, Sb, Bi, Pb, Sn, As, Si, P, O, N, In and mixturesthereof. Suitable chalcogenide switching materials include, but are notlimited to As₂Se₃ and other As—Se binary compositions; As₂Te₃ and otherAs—Te binary compositions; As₂₈Se₄₂Ge₃₀ and other As—Se—Ge ternarycompositions; As₃₂Te₃₆Ge₆Si₂₆, As₃₅Te₄₀Ge₇Si₁₈, and other As—Te—Ge—Siquaternary compositions, As_(31.75)Te₃₆Ge₆Si₂₆In_(0.25),As₃₅Te₄₀Ge_(6.75)Si₁₈In_(0.25) and other As—Te—Ge—Si quaternarycompositions containing In.

In composite switching devices such as those described herein,electrodes deliver an electric current to the composite chalcogenide orother switching material. As the electric current passes through acomposite chalcogenide material, at least a portion of the electricenergy of the electrons is transferred to the surrounding material asheat. A portion of the electrical energy of the current flowing throughthe composite chalcogenide material is converted to heat energy viaJoule heating. The amount of electrical energy converted to heat energyincreases with the resistivity of the electrical contact, theresistivity of the composite chalcogenide material and the currentdensity (i.e., current divided by area) passing through the switchingdevice.

As discussed, composite chalcogenide layer 150 is a mixture of achalcogenide material, such as those described above, and a dielectricmaterial. Generally, dielectric materials are materials which areelectrical insulators or in which an electric field can be sustainedwith a minimum dissipation of power. A solid is a dielectric if itsvalence band is full and is separated from the conduction band by atleast 3 eV.″ McGraw-Hill Encyclopedia of Physics, Second Edition, 1993,page 283. The dielectric materials used herein can be any dielectricmaterial that is chemically non-reactive with the chalcogenide material.Preferably, the dielectric material has a melting point higher than thatof the chalcogenide material.

In particular, the dielectric material may be one or more materialsselected from the group consisting of oxides, nitrides, fluorides,sulfides, chlorides, carbides, oxynitrides, carboxynitrides, borides,phosphides and mixtures or alloys thereof. Other dielectric materialsknown in the art may also be used. The dielectric material may also bechosen from the group of organic dielectric materials. These include,but are not limited to, materials such as amides, polyamides, imides,polyimides, parylenes and other high melting oligomeric or polymericorganic substances.

Oxides include silicon oxides such as SiO₂, titanium oxides such asTiO₂, aluminum oxides such as Al₂O₃, zirconium oxides such as ZrO₂,germanium oxides such as GeO₂, and tantalum oxides such as Ta₂O₅. Otherpossible oxides include B₂O₃, Sb₂O₃, PbO, and transition metal oxides.Suitable oxides include solid solutions and oxides including two or moremetal atoms in the composition. Nitrides include silicon nitrides suchas Si₃N₄, aluminum nitrides such as AlN, as well as TiN, SiN, ZrN, BN,CN, and off-stoichiometry silicon nitride SiN_(x). Sulfides includesilicon sulfide such as SiS₂, germanium sulfide such as GeS₂, and zincsulfide such as ZnS. Fluorides include MgF₂, CaF₂, and LiF₂. Variousglasses may also be used. For instance, LaSiON material containing La,Si, O and N; SiAlON material containing Si, Al, O and N; SiAlON materialdoped with yttrium; or NdSiON material containing Nd, Si, O and N may beused. Amorphous oxides, such as amorphous SiO₂, may also be used as adielectric in the instant composite switching material.

The composite switching material may be a chalcogenide materialincluding a heterogeneous mixture of active (switchable) chalcogenidematerial and inactive (non-switchable) dielectric material. Oneembodiment of such a heterogeneous mixture is that of a multi-layeredstructure with layers of chalcogenide material intermixed with layers ofdielectric material. The thickness of each layer, for example, may bebetween about 5 Å to about 75 Å. In another example, the thickness ofeach layer may be between about 10 Å to about 50 Å. Alternatively, thethickness of each layer may be between about 20 Å to about 30 Å. Thecomposite switching material may include a periodic or non-periodicarrangement of dielectric or other non-switchable material dispersedwithin an active chalcogenide or other switching material. The compositeswitching material may include a combination of multiple layers, some ofwhich include a dielectric material interspersed within a switchablematerial and some of which include only a switchable material or only adielectric material. A dual layer structure, for example, that includesa first layer comprising a heterogeneous combination of a dielectricmaterial and a switchable chalcogenide material and a second layercomprising a switchable or non-switchable chalcogenide material in theabsence of a dielectric material is within the scope of the instantinvention. A dual layer structure that includes two layers, each ofwhich includes a combination of a dielectric material interspersed witha switchable chalcogenide material, is also within the scope of theinstant invention.

The composite chalcogenide material may be made by methods such assputtering, evaporation, or by chemical vapor deposition (CVD), whichmay be enhanced by plasma techniques such as RF glow discharge. Thecomposite chalcogenide material may be, in an example, made by RFsputtering. It may be formed by multiple source sputtering techniquesthat make use of a plurality of targets, usually a target of thechalcogenide material and a target of the dielectric material. Withthese targets arranged in opposition to a substrate, sputtering may becarried out while the substrate is rotated relative to each target. Atarget containing both chalcogenide and dielectric materials may be usedas well. Substrate heating may be used to control the morphology of thechalcogenide material within the composite chalcogenide material formed.

The percentage by volume of dielectric material within the compositechalcogenide material can be controlled in the manufacturing process. Ina first example, the percentage by volume of the inactive dielectricmaterial to active chalcogenide or switching material is between about5% and 90%. In a second example, the percentage by volume of dielectricmaterial is between about 5% and about 20%. In a third embodiment, thepercentage by volume of dielectric material is between about 10% andabout 15%.

The composite chalcogenide material may also be formed by a spin coatingprocess. The chalcogenide material may be a heterogeneous mixture of achalcogenide material and a dielectric such as an organic polymer like apolyamide. The resulting mixture may then be spin coated onto a siliconsubstrate to form a composite chalcogenide material with the desiredproperties. A chalcogenide or switching material may be suspended withina solution phase and the resulting suspension spin-coated on a substrateor lower support structure to form a composite material.

FIG. 5A is a cross-sectional view of a prior art chalcogenide device 340lacking a dielectric component and having a first filament 350 inchalcogenide layer 370. In a first switching event, first filament 350is formed over a first path between the 2nd lower electrode 130 andupper electrode 170. A first lower contact location 355 is formed at theinterface of chalcogenide layer 370 and the 2nd lower electrode 130. Afirst upper contact location 360 is formed at the interface ofchalcogenide layer 370 and upper electrode 170.

FIG. 5B is a cross-sectional view of a prior art chalcogenide device 340lacking a dielectric component and having a second filament 380 inchalcogenide layer 370. In a second switching event, second filament 380is formed over a second path between the 2nd lower electrode 130 andupper electrode 170. A second lower contact location 385 is formed atthe interface of chalcogenide layer 370 and the 2nd lower electrode 130.A second upper contact location 390 is formed at the interface ofchalcogenide layer 370 and upper electrode 170.

Variations in the upper and lower contact locations of the filament andthe path over which the filament forms may arise due to variations inseveral factors, including: the properties of the chalcogenide switchingmaterial 370, the properties of the 2nd lower electrode contact 130 orits interface with chalcogenide material 370, the properties of uppercontact 170 or its interface with chalcogenide material 370, and/or heatdissipation effects within chalcogenide material 370 or the surroundingstructure during cycling, The filament formation process requiresapplication of at least a threshold voltage between the 2nd lowerelectrode contact 130 and upper contact 170 and is accompanied by heatgeneration that may lead to expansion or physical displacement of thechalcogenide material. When the composite switching material returnsfrom its conductive state back to its resistive state, the filamentcollapses and the material cools. The repeated heating and cooling ofthe material, along with any accompanying thermal and mechanicalstresses, may induce variations in the switching properties of thedevice. Such effects may lead to cycle-to-cycle variations in thresholdvoltage, threshold current, holding voltage, holding current, path offilament formation, and/or location of the points of contact of thefilament with upper and lower electrodes.

FIG. 6A is a cross-sectional view of a composite chalcogenide device 410having a first filament 420 formed in composite chalcogenide layer 150having relatively large insulative particles. In a first switchingevent, first filament 420 is formed. A first lower contact location 430is formed at the interface of composite chalcogenide layer 150 and the2nd lower electrode 130. A first upper contact location 432 is formed atthe interface of composite chalcogenide layer 150 and upper electrode170. As shown, first filament 420 takes a path through chalcogenidematerial 310 and avoids dielectric material 320.

FIG. 6B is a cross-sectional view of a composite chalcogenide device410′ having a second filament 460 formed in composite chalcogenide layer150 having the same relatively large insulative particles as in FIG. 6A.In a second switching event, second filament 460 is formed. A secondlower contact location 470 is formed at the interface of compositechalcogenide layer 150 and the 2nd lower electrode 130. A second uppercontact location 472 is formed at the interface of compositechalcogenide layer 150 and upper electrode 170.

In comparing FIGS. 6A and 6B, second filament 460 takes a different paththan first filament 420 through composite chalcogenide layer 150, butalso avoids dielectric material 320. Although first lower contactlocation 430 and second lower contact location 470 contact the 2nd lowerelectrode 130 at different locations, the presence of dielectric 320tends to constrain the variability of the point of contact. Similarly,first upper contact location 432 and second upper contact location 472contacting upper electrode 170, may occur at different locations, butare also influenced and constrained by the presence of dielectric 320.

Due to the presence of dielectric material 320 in the embodiments shownin FIGS. 6A and 6B, the volume fraction of chalcogenide material 310 isreduced relative to the chalcogenide-only device shown in FIGS. 5A and5B. The presence of the dielectric 320 acts to confine and channel thefilament through the chalcogenide component relative to thechalcogenide-only device and provides for greater reproducibility ofswitching performance over repeated cycles of operation. Since thefilament does not form through the dielectric, the presence ofdielectric 320 limits the range of paths over which the filament mayform during different cycles of operation.

FIG. 7A is a cross-sectional view of a composite chalcogenide device 510of FIG. 2 having a first filament formed in the composite chalcogenidematerial having relatively smaller insulative particles than the exampleof FIG. 6A. In the manufacturing process (see FIG. 10), relativelysmaller particles of dielectric material 514 may be deposited along withchalcogenide material 512. Here, the fineness of the insulativeparticles has the effect of improving the homogeneity of compositechalcogenide layer 150. Thus, the operating parameters of compositechalcogenide device 510 may be tuned for a desired performance parameterby adjusting the size of dielectric particles 514 as well as thedielectric material and chalcogenide material. Here, a first filament520 is formed between a first lower contact location 530 and a firstupper contact location 532.

FIG. 7B is a cross-sectional view of a composite chalcogenide device510′ having a second filament formed in the composite chalcogenidematerial having relatively small insulative particles. Here, a secondfilament 560 is formed between a second lower contact location 570 and asecond upper contact location 572. In comparing FIGS. 7A and 7B, secondfilament 560 takes a slightly different path than first filament 520through composite chalcogenide layer 150, but the overallreproducibility of filament formation and switching characteristics isexpected to improve relative to the chalcogenide-only device shown inFIGS. 5A and 5B.

FIG. 8 is a graph 600 describing leakage current vs. dielectric fractionfor the composite chalcogenide device 100 of FIG. 2. In this example, achalcogenide material (Te₄₂As₂₈Ge₃₀) is co-sputtered with silicondioxide (SiO₂) and annealed at 300° C. The data show the leakage currentas a function of volume fraction of SiO₂ in the composite chalcogenideswitching material at a device voltage of approximately half thethreshold voltage. Data were taken after 10⁶ cycles. FIG. 8 shows theaverage leakage current after 1e6 cycles as well as the range of leakagecurrents obtained on many devices. The data show a generally reducedleakage current with increased addition of SiO₂ above a volume fractionof 5% in composite chalcogenide layer 150 (see FIG. 4). Additionally,the variation of leakage current upon cycling decreases as the volumefraction of SiO₂ increases.

FIG. 9 is a graph 700 describing an example of life cycle vs. dielectricfraction for composite chalcogenide device 100 of FIG. 2. As shown, thelife cycle of composite chalcogenide device 100 increases as the volumefraction of SiO₂ in the composite chalcogenide layer 150 is increased.

FIG. 10 is a flow diagram 8000 of the construction of compositechalcogenide device 100 of FIG. 2. In step 8010, lower isolation layer102 is provided. Lower isolation layer is typically made of SiO₂(silicon dioxide) and is readily deposited by techniques such aschemical vapor deposition (CVD), plasma enhanced chemical vapordeposition (PECVD), sputtering or spin-casting. SiO₂ may also form as agrown oxide on silicon-based substrates. As is known in the art, silicondioxide is a common insulator in semiconductor device technology. Lowerisolation layer 102 provides electrical and thermal isolation from anyadjacent structures.

Next, in step 8020 lower electrode 110 is provided. Lower electrode 110is typically a tungsten (W) layer or refractory metal deposited bysputtering or CVD methods. As composite chalcogenide device 100 may beconstructed between steps in a semiconductor process, lower electrode110 may be deposited along with other interconnect lines for othercircuitry under construction.

Next, in step 8030 lower insulator 120 is provided. Lower insulator 120may also be a silicon dioxide material.

Next, in step 8040 lower insulator 120 is configured to create hole 122.In this step, a hole is etched through lower insulator 120 to exposelower electrode 110 using, e.g., reactive ion etching (RIE) or achemical etch with appropriate masking techniques such asphotolithography.

Next, in step 8050 the 2nd lower electrode 130 is provided. The 2ndlower electrode 130 is preferably a low electrical and low thermalresistance material such as W, TiN, TaN, WN, titanium silicon nitride,tantalum nitride, or other materials. The 2nd lower electrode 130 may beformed by sputtering (PVD), chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), atomic layer deposition(ALD), or other deposition method generally known in the art.

Next, in step 8060, excess the 2nd lower electrode material is removed.The surface is generally planarized using chemical mechanical polishing(CMP) techniques to provide a planer surface for composite chalcogenidelayer 150.

Next, in step 8070 composite chalcogenide layer 150 is provided.Co-sputtering of chalcogenide materials with various dielectricmaterials can be used to reduce the volume fraction of active switchingmaterial as described hereinabove. In an embodiment, an OTS compositionas described hereinabove is co-sputtered with SiO₂ to form compositechalcogenide layer 150. Using co-sputtering composite chalcogenide layer150 is heterogeneously a mixture of chalcogenide threshold alloy andinsulator. Another deposition technique is to use sputtering of aheterogeneous target that contains chalcogenide material and insulatormaterial.

Next, in step 8080 upper electrode 170 is provided. Typically, upperelectrode 170 is metallic and is deposited by sputtering or evaporation.

Next, in step 8090 capping insulator 180 is provided for isolation ofcomposite chalcogenide device 100. Capping insulator 180 may comprise amaterial such as SiO₂ or Si₃N₄. In a preferred embodiment, silicondioxide is used.

The present invention has been particularly shown and described withreference to the foregoing embodiments, which are merely illustrative ofthe best modes for carrying out the invention. It should be understoodby those skilled in the art that various alternatives to the embodimentsof the invention described herein may be employed in practicing theinvention without departing from the spirit and scope of the inventionas defined in the following claims. The embodiments should be understoodto include all novel and non-obvious combinations of elements describedherein, and claims may be presented in this or a later application toany novel and non-obvious combination of these elements. Moreover, theforegoing embodiments are illustrative, and no single feature or elementis essential to all possible combinations that may be claimed in this ora later application.

With regard to the processes, methods, heuristics, etc. describedherein, it should be understood that although the steps of suchprocesses, etc. have been described as occurring according to a certainordered sequence, such processes could be practiced with the describedsteps performed in an order other than the order described herein. Itfurther should be understood that certain steps could be performedsimultaneously, that other steps could be added, or that certain stepsdescribed herein could be omitted. In other words, the descriptions ofprocesses described herein are provided for illustrating certainembodiments and should in no way be construed to limit the claimedinvention.

Accordingly, it is to be understood that the above description isintended to be illustrative and not restrictive. Many embodiments andapplications other than the examples provided would be apparent to thoseof skill in the art upon reading the above description. The scope of theinvention should be determined, not with reference to the abovedescription, but should instead be determined with reference to theappended claims, along with the full scope of equivalents to which suchclaims are entitled. It is anticipated and intended that futuredevelopments will occur in the arts discussed herein, and that thedisclosed systems and methods will be incorporated into such futureembodiments. In sum, it should be understood that the invention iscapable of modification and variation and is limited only by thefollowing claims.

All terms used in the claims are intended to be given their broadestreasonable constructions and their ordinary meanings as understood bythose skilled in the art unless an explicit indication to the contraryis made herein. In particular, use of the singular articles such as “a,”“the,” “said,” etc. should be read to recite one or more of theindicated elements unless a claim recites an explicit limitation to thecontrary.

1. An electrical device comprising: a first electrode; a secondelectrode; and a composite switching material disposed between saidfirst electrode and said second electrode, said composite switchingmaterial including a switchable component and a non-switchablecomponent, said switchable component having a resistive state and aconductive state, said switchable component switching from saidresistive state to said conductive state upon application of a voltagebetween said first electrode and said second electrode.
 2. Theelectrical device of claim 1, wherein said switchable component and saidnon-switchable component form a heterogeneous layer between said firstelectrode and said second electrode.
 3. The electrical device of claim1, wherein the volume fraction of said non-switchable component in saidcomposite switching material is between 5 percent and 90 percent.
 4. Theelectrical device of claim 1, wherein the volume fraction of saidnon-switchable component in said composite switching material is between5 percent and 20 percent.
 5. The electrical device of claim 1, whereinsaid switchable component comprises a chalcogenide material.
 6. Theelectrical device of claim 5, wherein said chalcogenide materialcomprises Te or Se.
 7. The electrical device of claim 6, wherein saidchalcogenide material further comprises Ge and As.
 8. The electricaldevice of claim 1, wherein said non-switchable component comprises adielectric material.
 9. The electrical device of claim 8, wherein saiddielectric comprises a material selected from the group consisting ofoxides, nitrides, carbides, oxynitrides, and borides.
 10. The electricaldevice of claim 9, wherein said dielectric material comprises silicon.11. The electrical device of claim 1, wherein said switchable componentis in said resistive state when the voltage applied across said firstelectrode and said second electrode is less than a first voltage. 12.The electrical device of claim 11, wherein said switchable componentswitches to said conductive state when the voltage applied across saidfirst electrode and said second electrode is equal to or greater thansaid first voltage, said device conducting at least a first currentbetween said first electrode and said second electrode in saidconductive state.
 13. The electrical device of claim 12, wherein saidswitchable component returns to said resistive state when the currentpassing between said first electrode and said second electrode is lessthan said first current.
 14. The electrical device of claim 1, whereinsaid conductive state of said switchable component comprises aconductive filament, said conductive filament extending from said firstelectrode to said second electrode.
 15. The electrical device of claim14, wherein said conductive state of said switchable component comprisesa first region and a second region, said first region comprising saidconductive filament, said second region comprising said switchablecomponent in said resistive state.
 16. The electrical device of claim 1,further comprising an insulating layer disposed between said firstelectrode and said second electrode.
 17. The electrical device of claim16, wherein said insulating layer includes a hole, said first electrodeoccupying said hole.
 18. The electrical device of claim 1, wherein saidfirst electrode contacts said composite switching material.
 19. Theelectrical device of claim 18, wherein said second electrode contactssaid composite switching material.
 20. The electrical device of claim19, wherein the area of contact of said first electrode with saidcomposite switching material is less than the area of contact of saidsecond electrode with said composite switching material.
 21. Theelectrical device of claim 2 further comprising a second layer, saidsecond layer disposed between said heterogeneous composite switchinglayer and said first electrode.
 22. The electrical device of claim 21,wherein said second layer comprises a chalcogenide material.
 23. Theelectrical device of claim 22, wherein said second layer furthercomprises a dielectric material dispersed within said chalcogenidematerial.
 24. The electrical device of claim 22, wherein saidchalcogenide material is a switchable chalcogenide material.